The present invention relates to the field of heat exchangers.
It is known to provide heat exchangers for heat transfer between a first fluid and a second fluid, comprising:
a central casing of generally axisymmetric cylindrical shape formed by two end walls and a cylindrical peripheral wall equipped with inlet and outlet piping for the first fluid;
a bank of tubes which are elongated parallel to the axis of the casing, the end regions of each tube passing, in fluid-tight fashion, through apertures formed in said end walls, and the outer faces of the tubes delimiting, within the casing, a chamber for the circulation of the first fluid; and
two axisymmetric annular caps respectively capping the two ends of the casing and connected in a leak tight way at their periphery, so as, with the end manifolds which communicate with each other by means of tubes for the circulation of the second fluid, to define the central apertures of the caps constituting respectively an inlet aperture and an outlet aperture for the second fluid.
Such exchangers are used particularly for recovering heat from the exhaust gases of motor vehicles, the first fluid being a fluid in circulation for recovering the heat, and the second fluid consisting of the exhaust gases.
In these known exchangers, the various elements are assembled together so as to be fluid-tight by brazing, and the end caps are solid molded components. The material used is stainless steel, in order to allow brazing. These pieces are re-worked at the machining stage in order to obtain a good fit between the assembly and the other components and to obtain the correct surface finish necessary for brazing. The material and the manufacturing technique make the pieces heavy and expensive.
Moreover, the caps have no elasticity and have to be fitted over the casing with clearance. They cannot be held on the casing by friction, and tack welds have to be formed in order to hold the pieces together until brazing.
Finally, the absence of elasticity of the caps severely limits the permitted oval shaping of the casing.
An object of the invention is to remedy the drawbacks set out above.
According to a first aspect the present invention provides a heat exchanger for heat transfer between a first fluid and a second fluid, comprising:
a central casing of generally axisymmetric cylindrical shape formed by two end walls and a cylindrical peripheral wall equipped with inlet and outlet piping for the first fluid and by two end walls;
a bank of tubes which are elongated parallel to the axis of the casing, the end regions of each tube passing, in fluid-tight fashion, through apertures formed in said end walls, and the outer faces of the tubes delimiting, within the casing, a chamber for the circulation of the first fluid; and
two axisymmetric annular caps respectively capping the two ends of the casing and connected in a leak tight way to their periphery, so as, with the end walls of manifolds which communicate with each other by means of tubes for the circulation of the second fluid, to define central apertures of the caps constituting respectively an inlet aperture and an outlet aperture for the second fluid, wherein the caps are made from stamped sheet metal.
The stamped sheet metal caps are lighter and less expensive than the molded and machined caps of the state of the art. Their elasticity facilitates assembling the caps the casing and holding them in the assembled state before brazing.
These effects are further reinforced by a slight oval shaping, obtained naturally by the stamping from thin rolled sheet metal. The oval shape and sheet metal material make it possible simultaneously to obtain an appropriate diametrical clearance between the casing and the cap, and light clamping in a particular radial direction, allowing the assembly to be held together.
Optional supplementary or alternative characteristics of the invention are set out below:
at least one of the caps include, at its axial end turned away from the casing, a radially internal cylindrical portion defining axial inlet or outlet piping for the second fluid.
the outer face of the axial piping is brazed to the inside of a through hole formed in a plate-type fixing flange which extends perpendicularly to the axis of the casing.
the cap includes, close to the axial piping, a portion oriented substantially radially which is brazed onto the face of the flange turned towards the casing.
at least one of the caps includes a snug-fitting cylindrical portion covering an end region of said peripheral wall.
said snug-fitting cylindrical portion is adjacent to a flared portion situated at the axial end of the cap turned towards the casing, able to facilitate the fitting of the cap snugly on the casing.
said snug-fitting cylindrical portion is connected, going away from the casing, to a portion of progressively decreasing diameter forming an abutment for the end of the casing and defining a path of variable cross-section for the second fluid between the casing and the inlet or outlet aperture.
According to another aspect, the invention provides a method of assembling a heat exchanger comprising the steps of:
assembling a central casing of generally axisymmetric cylindrical shape formed by two end walls and a cylindrical peripheral wall equipped with inlet and outlet piping for the first fluid;
assembling a bank of tubes which are elongated parallel to the axis of the casing, the end regions of each tube passing, in fluid-tight fashion, through apertures formed in said end walls, and the outer faces of the tubes delimiting, within the casing, a chamber for the circulation of the first fluid; and
assembling two axisymmetric annular caps respectively capping the two ends of the casing and connected in a leak tight way to their periphery, so as, with the end walls of manifolds which communicate with each other by means of tubes for the circulation of the second fluid, to define central apertures of the caps constituting respectively an inlet aperture and an outlet aperture for the second fluid, wherein the caps are made from stamped sheet metal and wherein at least one of the caps exhibits a cylindrical snug-fitting portion covering an end region of said peripheral wall in which method:
after having assembled the casing and the tubes, the caps are fitted snugly over the end regions of the casing, and the assembly thus obtained is brazed. The external diameter of the casing, the internal diameter of the snug-fitting cylindrical portion and the elasticity of the caps is such that the caps fit snugly with friction holding them in place before the brazing.
The method according to the invention may include at least some of the following features:
the casing and/or the caps exhibit a slightly oval shape and have an external diameter and an internal diameter respectively; the external diameter having a minimum value less than the minimum value of the internal diameter and having a maximum value greater than the maximum value of the internal diameter and the internal diameter of said snug-fitting portion being greater than the external diameter of the casing.
the snug-fitting movement of the caps is continued until their portion of progressively decreasing diameter comes into abutment on the end of the casing.